Collaborative Research: Carbonation of Serpentinite in the San Andreas Fault: How Fluid-rock Interactions Impact Aseismic Creep

众所周知，圣安德烈亚斯断层的某些部分经常发生小规模地震，偶尔发生中等规模地震，以及地震蠕变。其他部分没有地震蠕变，但罕见的致命破裂，如1857年和1906年的地震。帕克菲尔德北部圣安德烈亚斯断层沿着的高速率的地壳蠕变最好用大量热液蚀变的海岸山脉地幔岩的低剪切强度来解释。虽然蛇纹石，水化地幔岩的主要矿物，机械强度太强，不允许观察到的高位移速率，但人们越来越认识到滑石在削弱圣安德烈亚斯断层中起着关键作用。然而，在圣安德烈亚斯断层滑石形成的机制，条件和后果仍然存在争议。本项目中待检验的假设是，Cholame Valley和圣胡安鲍蒂斯塔之间高达28 mm/年的高蠕变速率受富含二氧化碳的流体与蛇纹石反应形成机械强度较弱的滑石和菱镁矿（皂石）的控制。此外，据推测，皂石与富含二氧化碳的流体长时间的相互作用导致形成机械更强和速度减弱的石英（或蛋白石）和菱镁矿，促进微地震在名义上弱蛇纹岩。为了检验这两种假设，该项目将把岩相学结果与流体包裹体分析和摩擦实验相结合，以确定：1）矿物置换反应，2）形成条件，3）在Cholame和圣胡安鲍蒂斯塔之间逐渐发生二氧化碳蚀变的海岸山脉岩石的地质力学特征。最后，三维随时间变化的地震活动性数据沿着SAF北部的帕克菲尔德将进行分析，以检查其与矿物学zoning.加州经历了致命的，高影响的地震，夺去了数百人的生命，造成数十亿美元的损失，因此有一个关键需要了解流体-岩石相互作用如何影响的力学性能的圣安德烈亚斯断层系统。这个项目探索了一个新的想法，可以解释为什么断层的某些部分表现出地震蠕变，而其他部分则以地震为特征。最终，这项对蛇纹岩碳酸化作用的研究将促进对美国人口最稠密地区之一的地震活动性和地震变形周期的更深入了解。

It is well established that some segments of the San Andreas Fault reveal frequent earthquakes of small magnitude, occasional earthquakes of moderate magnitude, and aseismic creep. Other segments feature no aseismic creep, but rare fatal ruptures, such as the 1857 and 1906 earthquakes. The high rates of aseismic creep along the San Andreas Fault north of Parkfield are best explained by the low shear strength of abundant hydrothermally altered Coast Range mantle rocks. While serpentine, the dominant mineral of hydrated mantle rock, is mechanically too strong to allow the observed high displacement rates, there is a growing recognition that talc plays a key role in weakening the San Andreas Fault. Yet the mechanisms, conditions and consequences of talc formation in the San Andreas Fault remain controversial. The hypothesis to be tested in this project is that the high creeping rates of up to 28 mm/yr between Cholame Valley and San Juan Bautista are controlled by the reaction of carbon dioxide-rich fluids with serpentine to form mechanically weak talc and magnesite (soapstone). In addition, it is hypothesized that prolonged interaction of soapstone with carbon dioxide-rich fluids causes the formation of mechanically much stronger and velocity-weakening quartz (or opal) and magnesite, promoting micro-earthquakes in a nominally weak serpentinite. To test both hypotheses this project will merge petrographic results with fluid-inclusion analysis and friction experiments to determine: 1) mineral replacement reactions, 2) formation conditions, and 3) geomechanical characteristics of progressively carbon dioxide-altered Coast Range rocks cropping out between Cholame and San Juan Bautista. Finally, three-dimensional time-dependent seismicity data along the SAF north of Parkfield will be analyzed to examine its correlation with mineralogical zoning.California has endured fatal, high-impact earthquakes, which claimed hundreds of lives and caused billions of dollars of damage; hence there is a critical need to understand how fluid-rock interactions influence the mechanical properties of the San Andreas Fault system. This project explores a new idea that might explain why certain portions of the fault exhibit aseismic creep whereas other portions are characterized by earthquakes. Ultimately, this study of carbonation of serpentinite will promote a deeper understanding of the seismicity and earthquake deformation cycles in one of the most densely populated regions of the United States.